Optical pH sensors have become very popular analytical tools in the past, due to their broad application in marine research, blood measurement, toxicological assay, and biotechnology. Generally, the fluorophore indicators are immobilized in a matrix for the purpose of obtaining optical pH sensors with stable lifetime and signal. Most optical pH indicators are immobilized in a suitable, proton-permeable, sol-gel polymer matrix. However, some inherent demerits of polymers, such as relatively poor thermal or optical stability as well as toxicity, have limited the practical application of pH sensors to date. Therefore, it is essential to search for novel materials to immobilize the fluorophore indicator in order to achieve optical pH sensors with high stability and environmental compatibility.
Recently, considerable interest has been focused on the fabrication of chromophore-inorganic composite materials, since they may show novel functionalities that are not present in the individual components alone. Among inorganic matrices, layered double hydroxides (LDHs) are one type of important layered materials that display a large versatility in terms of their chemical composition and the ability to build up 2D-organized structures, which show great application. The incorporation of chromophores into the layered double hydroxide (LDH) gallery exhibits the following advantages: First, the LDH matrix provides chromophore molecules with a confined and stable environment, which is the most necessary condition for the solid dye device. Second, chromophore aggregation in the LDH matrix is effectively inhibited by host-guest interactions (e.g., electrostatic attraction, hydrogen bonding), and fluorescence quenching is therefore reduced; Third, chromophore molecules immobilized in the LDH matrix exhibit optical and thermal stability, environmental compatibility, and low operational risk; (4) The LDH can be prepared into gel solution, and is more convenience for pH sensor.
Many preparation methods of LDH thin films have been reported, such as solvent evaporation, layer-by-layer technique, and electrophoretic deposition (EPD) method. However, this technique has recently gained in interest compared with the other methods such as their weak adherence and lower thickness, EPD has its own advantage: 1) easily to control the thickness from nano to micrometer; 2) high deposition rate and continuously. 3) it can make the film on any shape substrate; 4) strong adherence to substrates.